The Global Positioning System (GPS) technology is already in mobile phones in the USA, as part of the E911 requirement, and it will be coming to mobile phones in other continents in the near future too, but probably more for navigational aids rather than emergency call positioning. The idea behind the E911 system in the USA is to provide the emergency services with your location data when you are phoning from a mobile phone, say for example, from the side of the road because your car has broken down. The mobile phone sends your precise location information (50-300 meters accuracy) to a local Public Safety Access Point (PSAP), including your mobile phone number. This enables the emergency services to send the correct services (for example, fire, police or ambulance/medical) and more importantly to the correct location. For navigational aids, it is a different matter, as the user is now using the product for recreational purposes rather than emergency situations. The product is then more likely to be used in the hand with the liquid crystal display (LCD) screen facing the user, whereas in an E911 usage scenario, the user will have the product against their head making a phone call, while also receiving GPS data. The operational modes for visual-based location applications and that for emergency positioning can therefore be quite different.
In GPS positioning, multipath signals are one of the major sources of errors for positioning accuracy, but this is not directly related to polarization losses or antenna performance. A GPS signal is right-hand circularly polarized (RHCP), and the polarization may change when the signal is reflected. Good location accuracy expects that the antenna has a clear view of the GPS satellites and a good RHCP radiation pattern toward the sky.
It is desirable to provide an antenna that can provide an omni-directional RHCP radiation pattern. While right-hand circular polarization alone is important, it is also of importance to have a GPS antenna which can be used when the product is held in the hand by the user, and when the product is held against the head during a phone call. In order to meet both of these product use cases we need an antenna design which can “see” satellites wherever and however the product is used by the user. This calls for an omni-directional radiation pattern, in other words, the antenna radiates in all directions from the product, and therefore is able to obtain a clear view of the GPS satellites, while maintaining the RHCP properties of the antenna. Traditional GPS receivers (not mobile phones) implement antennas which are RHCP, but have only hemispherical coverage, this means they can “see” the sky. If the product is tilted, then the antenna may see fewer satellites as a consequence. The other phenomena is that when the mobile phone is placed against the user's head in an E911 call, then the radiation pattern of the GPS antenna will be affected due to the presence of the head, and this can also lead to loss of satellites. Typically omni-directional antennas are linearly polarized, but in this invention both RHCP (circular polarization) and omni-directional radiation has been achieved. The antenna is a significant part of any radio communication system. Signal processing can not compensate for a poor antenna, especially in satellite communications. It is not exaggerated at all to say that the antenna is the key component in the GPS receiving system.